Control system for air/fuel ratio adjustment system

ABSTRACT

The invention is concerned with a control system for an air/fuel ratio measurement and adjustment system for an internal combustion engine. In order to restrict operation of the air/fuel ratio measurement and adjustment system to occasions when adjustment is likely to be necessary, it is desirable to operate the system at approximately daily intervals. The control system incorporates an ambient temperature sensor and an engine coolant temperature sensor and is arranged to operate the measurement and adjustment system when the engine is started if the engine coolant temperature does not exceed the ambient temperature by more than a predetermined amount.

The invention relates to control systems for air/fuel ratio adjustmentsystems for internal combustion engines. It has already been proposed,for example in our co-pending UK Patent Application No. 8216444, tomeasure the air/fuel ratio at which an internal combustion engine isoperating by detecting and measuring combustible orcombustion-supporting constituents in the exhaust gas. This measure isthen used to effect an adjustment to bring the air/fuel ratio to adesired level. In this process of detecting combustion-supportingconstituents, an auxiliary fuel such as butane is used. It is desiredthat the consumption of the auxiliary fuel should be kept to a minimumto avoid continuous replacement of small auxiliary fuel canisters andfor that reason it is desired to control the operation of the air/fuelratio measurement and adjustment system in such a way that themeasurement system is operated only when there is a reasonablelikelihood that adjustment will be required. In principle, it isdesirable to operate the measuring and adjustment system once per daywhen the internal combustion engine is in use and also to operate themeasurement and adjusting system when a vehicle in which the engine isinstalled undergoes a substantial change in altitude. A change inambient pressure associated with a change in altitude can result in achange in air/fuel ratio and thus a requirement for adjustment.

An object of the present invention is to provide a control system for anair/fuel ratio measurement and adjustment system for an internalcombustion engine which operates at approximately daily intervals.

Our co-pending patent application no. filed on the same date as thepresent application is concerned with corresponding operation inresponse to changes in altitude.

According to the invention there is provided a control system for anair/fuel ratio measurement and adjustment system for an internalcombustion engine, the control system comprising: an ambient temperaturesensor, an engine coolant temperature sensor and means for initiatingmeasurement of the air/fuel ratio in response to the engine coolanttemperature not exceeding ambient temperature by more than apredetermined amount. During normal use of a vehicle several occasionsduring a day, the coolant temperature drops slowly towards ambienttemperature while the engine is not operating but will take severalhours to approach within say 5° of ambient temperature. Thus if avehicle is used several times during a day, the coolant temperature doesnot approach close to ambient temperature during the day. In contrast,when a vehicle is left overnight the coolant temperature dropssubstantially to ambient temperature and as ambient temperature rises inthe morning the coolant temperature may be below ambient temperature.Thus by effecting an air/fuel ratio measurement only when coolanttemperature approaches or is less than ambient temperature, themeasurements are effected approximately once per day.

Preferably the control system also comprises means for measuring theextent of engine inlet throttle opening, engine speed and absolute inletmanifold pressure, means for deriving from stored information anexpected inlet manifold pressure to correspond to the measured throttleopening and engine speed, means for deriving and storing the ratiobetween the expected and actual manifold pressures, means for comparinga subsequently derived ratio between expected and actual manifoldpressures with a previously determined such ratio and means forinitiating measurement of the air/fuel ratio in response to apredetermined extent of change in said pressure ratio. In this way, apressure sensor for inlet manifold pressure, which is also needed forother purposes, serves as the basis for providing a measure of ambientpressure and thus avoids the need for an independent ambient pressuresensor.

Preferably the control system also incorporates means for detectingengine operating parameters capable of indicating vehicle operation in anormal cruising mode and means for preventing initiation of air/fuelratio measurement except when the vehicle is operating in a normalcruising mode.

An embodiment of the invention will be described with reference to theaccompanying drawings in which:

FIG. 1 is a block diagram showing the fundamental units employed in thepresent invention including a central processor unit; and

FIG. 2 is a flow chart indicating the nature of the operations carriedout in the central processor unit.

As shown in FIG. 1, a central processor unit constituted by a digitalmicro-processor is connected to receive signals from the followingsources:

(a) An engine coolant temperature sensor. This may be the same sensor asis used to give the vehicle driver an indication of coolant temperaturealthough an accurate sensor capable of measuring changes of the order of2° C. is required.

(b) An ambient temperature sensor. This requires similar accuracy to thecoolant temperature sensor and should be disposed on the vehicle awayfrom sources of local heat such as exhaust systems, brakes and theengine itself but also in a position where extensive solar heating abovenormal ambient temperature will not occur.

(c) A throttle angle sensor. This unit is required to measure the degreeof throttle opening and in the case of a sliding throttle would notmeasure angle as such.

(d) An engine speed sensor.

(e) An absolute inlet manifold pressure sensor. In an accuratelycontrolled fuel metering system for an internal combustion engine, thefuel supply rate is controlled directly in accordance with absolutemanifold pressure so such a pressure sensor is a normal part of the fuelsystem.

The central processor unit also has access to stored data relatingexpected manifold pressure to engine speed and throttle opening whenoperating at sea level at normal atmospheric pressure. This data may forexample be established by calibration of a test engine. Recalibration isrequired only after substantial altitude changes of say 300 meters soonly a limited amount of data need be stored to give a coarsemeasurement of ambient pressure change. In response to supply from thecentral processor unit of given values of engine speed and thottleopening, this unit supplies the central processor unit with an expectedvalue for manifold pressure.

The central processor unit also has an output leading to an air/fuelratio sensor. This may be a sensor which measures the degree of leannessof the mixture by combusting an auxiliary fuel in the exhaust gases andmeasuring the heating effect of this combustion. The output from thecentral processor unit to the air/fuel ratio sensor simply initiatesoperation of the air/fuel ratio sensor. An indication of the air/fuelratio sensed is supplied to the central processor unit. If appropriate,the central processor unit then provides a signal to an air/fuel ratiocontrol to adjust the air/fuel ratio to a desired level.

The basic operations occurring within the central processor unit willnow be described with reference to FIG. 2. On starting the vehicleengine, the coolant temperature is compared with ambient temperature andif coolant temperature is not more than t°C. above ambient temperaturethe process proceeds to point A at which point an air/fuel ratiomeasurement is initiated as will be described subsequently.

If coolant temperature is above ambient by more than t°C., the systemgoes into the altitude sensing mode. Throttle angle, engine speed andmanifold pressure are measured and the calibrated manifold pressure forthe measured engine speed and throttle opening is looked up. The ratiobetween measured manifold pressure and the stored calibrated value isderived as a percentage to give a measure of current ambient pressure.This percentage is compared with a previously recorded and stored valuefor this percentage ratio, representative of the ambient pressure atwhich the last calibration took place. There is a delay of approximately200 seconds inherent in this comparison. If this change exceeds a givenpressure difference ΔP, the process again proceeds to point A. In theabsence of a substantial pressure change, the initial monitoring processis repeated at approximately 200 second intervals until a substantialpressure change, indicative of a change in altitude, is measured.

Once point A is reached, the following checks are carried outsuccessively:

(i) That coolant temperature is up to substantially normal runningtemperature and in particular is above t₂ °C.

(ii) That the inlet manifold pressure is within a range indicative ofoperation of the vehicle in a normal cruising mode.

(iii) That the engine speed is within a range indicative of vehicleoperation in a normal cruising mode.

(iv) That the throttle opening is within a range indicative of vehicleoperation in a normal cruising mode.

(v) That the rate of change of throttle opening is below a predeterminedlevel. Rather than measuring a rate of change as such, it is convenientto sense throttle opening at regular predetermined frequent intervals,to compare the current reading with an immediately preceding reading andassume a low rate of change when the difference between the two readingsis less than a predetermined amount.

When the results of all five checks are affirmative, calibration isinitiated. That is the air/fuel ratio sensor is brought into operationand if the measured ratio differs substantially from a desired storedvalue the air/fuel ratio control is operated to vary the air/fuel ratio.

If the result of one of the five checks (i) to (v) above is negative andno previous calibration has been effected since the vehicle was started,the operation reverts to point A and the process from there is repeated.

Immediately after calibration has been effected or if the varioussensors indicate that the vehicle is not in its normal cruising mode andthere has been a previous calibration, the control reverts to itsstarting point.

It can be seen that whenever the engine coolant temperature is low onstarting the engine calibration will be carried out as soon as normalcruising conditions are reached. This will not be until the coolanttemperature has reached a normal running level. If the coolanttemperature is above the low level of t°C., (i.e. above the temperaturewhich suggests the start of a new day) when the engine is switched on, acomparison of current and previous ambient pressures is carried out anda calibration then follows if but only if there has been a substantialchange in ambient pressure since the last calibration. In this way,calibration is effected when it is likely to be needed but the number ofcalibrations is very much restricted with the result that the auxiliaryfuel supply used during calibration has a long life.

We claim:
 1. A control system for an air/fuel ratio measurement andadjustment system for an internal combustion engine, the control systemcomprising: an ambient temperature sensor, an engine coolant temperaturesensor and means for initiating measurement of the air/fuel ratio inresponse to the engine coolant temperature not exceeding ambienttemperature by more than a predetermined amount.
 2. A control system asclaimed in claim 1 wherein the control system comprises: means formeasuring the extent of engine inlet throttle opening, engine speed andabsolute inlet manifold pressure; means for deriving from storedinformation an expected inlet manifold pressure to correspond to themeasured throttle opening and engine speed; means for deriving andstoring the ratio between the expected and actual manifold pressures,means for comparing a subsequently derived ratio between the expectedand actual manifold pressures with a previously determined such ratioand means for initiating measurement of the air/fuel ratio in responseto a predetermined extent of change in said pressure ratio.
 3. A controlsystem as claimed in claim 1 wherein prior to initiating measurement ofthe air/fuel ratio a check is carried out to establish that the vehicleis operating in a normal cruising mode and measurement is initiated onlyduring such a normal cruising mode.
 4. A control system as claimed inclaim 3 wherein the factors measured to establish operation in a normalcruising mode include engine speed, manifold pressure and degree ofthrottle opening.
 5. A control system as claimed in claim 4 wherein thefactors also include engine coolant temperature.
 6. A control system asclaimed in claim 4 wherein the factors also include rate of change ofthrottle opening.